A synchronous AC motor typically utilizes rotor position sensors to provide information regarding the position of the motor's rotor with respect to the motor's stator windings. Such positional information allows for proper conversion of power that is supplied to the stator windings. Rotor position sensors such as Hall effected devices are typically mounted in the stator, proximate the stator winding, to provide intelligence regarding rotor position. Such rotor position sensors, however, can be unreliable due to mechanism alignment problems and temperature incompatibility problems between the stator windings and electronic components such as the Hall effect devices. Moreover, the rotor position sensors can be difficult to mount to the motor during motor assembly, especially for multi-pole motors. In multi-pole motors, the electrical misalignment angle is equivalent to the angular mechanical misalignment angle multiplied by the number of pole pairs.
Due these and other drawbacks, sensorless techniques have been developed to determine rotor position. One sensorless rotor position detection technique observes back EMF voltages at the stator windings. Another technique, which applies a floating frame control (FFC) scheme, has been described by Huggett et al. in U.S. Pat. No. 6,301,136, which in hereby incorporated herein by reference in its entirety. In the FFC scheme, motor phase-current is detected directly and used to estimate rotor speed/position, and also to control the reactive current to zero. More specifically, sensorless rotor speed/position detection is combined with current control to achieve a closed-loop equilibrium condition in which an inverter voltage vector (Vωt) finds a position that results in a zero direct-axis current component value. Under this condition, a reference frame (floating frame) is synchronized with the magnetic axis of the rotor and can be used to control power conversion.
Such control results in unity power factor during steady state operation, which is an advantage for high power inverter design. Although the FFC scheme disclosed in U.S. Pat. No. 6,301,136 is effective in many applications and conditions, the speed/position estimation in the FCC scheme is embedded in the direct-axis current regulator, which makes the loop tuning sensitive in some applications.